You cannot say, with 100% certainty, that your student will pass the Skills Test. If the student should fail the test and remedial training is recommended by the examiner, are you really going to authorise his/her solo return to your operating airfield?
I don't see the difference with the first solo XC flight. In my opinion that's a far more dangerous flight from a skills perspective.
The only real risk I see with letting him fly back solo again after failure is the psychological state. He's probably sad and concentration might not be optimal in such situation.